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GCEP research team discovers improved energy conversion using ultra-low work function materials

The work function is the surface property of a material that determines how easily electrons can escape into a vacuum and facilitates electron emission. Ultra-low work function materials are critical and desirable for a type of energy conversion and electron-emission application described as thermionic. Thermionic-emission converter technologies can potentially use waste heat to generate electricity.

GCEP investigator Roger Howe (left) and Ph.D. candidate Hongyuan Yuan
GCEP investigator Roger Howe (left) and Ph.D. candidate Hongyuan Yuan

Now a GCEP-supported research group from Stanford has developed a conductive material with the lowest work function reported to date. The work involves surface engineering graphene, a two-dimensional carbon material, as well as creating a large voltage bias through a process called electrostatic gating. The research is led by Roger Howe, a professor of electrical engineering, and Hongyuan Yuan, a Ph.D. candidate in physics.

According to their study in the journal Nano Letters, this approach can significantly change the work function of graphene. Coated with a single layer of cesium and oxygen (Cs/O) atoms, the graphene work function drops to nearly 1 eV. Put in context, a thermionic emission converter equipped with a 1eV work-function anode is expected to convert thermal energy to electricity at an efficiency of almost 40%, compared to less than 10% efficiency if the anode work function is 2 eV.

“Having thermal energy being one of the most abundant and cheapest energy sources, the new material system we find may re-spark the future for thermionic emission,” said Yuan, the first author in the study.

Courtesy: Howe Lab
Schematic sketch of the back-gated device structure.
Schematic sketch of the graphene system with electrostatic gated structure and Cs/O surface dipole.

Piero Pianetta’s group at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory contributed to the work. Stanford contributors include Nick Melosh’s group in the Department of Materials Science and Engineering; Z.X. Shen’s group in the Department of Physics and Applied Physics; and Eric Pop’s group in the Department of Electrical Engineering.

December 11, 2015

GCEP Fact Sheet:
Using First-principles Simulations to Discover Materials with Ultra-low Work Functions for Energy Conversion Applications



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